The lateral pressure induced upon soil under a vertical applied pressure is an important and fundamental aspect of engineering behavior of the soil particularly in relationship to the pressure on retaining walls and to the soil-bearing capacity under loads. For example, assuming that the lateral pressure for a particular soil equals one-half of the vertical pressure; a vertical loading of 100 p.s.i. will cause a lateral pressure of 50 p.s.i. on an adjacent retaining wall, the ratio of lateral to vertical pressure in this case being one-half. Thus, the adjacent retaining wall must be able, at a minimum, to withstand a lateral pressure equal to one-half the vertical stress from the weight of the soil, which of course increases with depth. The relationship to bearing capacity of a given soil is somewhat more complicated than the above example since the lateral pressure tends to cause adjacent underlying soil to be displaced laterally which can induce a bearing capacity failure or what is termed "rutting". This is true because, when the adjacent soil is displaced laterally, soil adjacent to the laterally displaced soil develops its own minor principal stress as an uplift pressure that may exceed the restraining weight of the adjacent overburden soil.
As one can see, the importance of monitoring the ratio of lateral stress to vertical stress is of critical importance for placing any structure upon the soil. It is therefore of value to assess this relationship and to evaluate its effect for a given soil sample in order to know whether or not the soil can properly withstand the applied vertical pressure. The ratio of lateral to vertical stress is known as the Rankine stress ratio and is termed "K".
It is an object of this invention to provide a soil testing device which can continually monitor the Rankine stress ratio on a soil sample, as vertical pressure is increased, in order to determine the lateral stresses induced by a given vertical applied pressure in adjacent soil.
An additional object of this invention is to provide means for monitoring and adjusting lateral restraint on the sample, in order to more closely simulate field behavior of soil surrounded by identical soil.
An additional object of this invention is to provide a Rankine stress monitoring device which may be used either on undisturbed field samples or soil samples which have been subjected to pre-treatment in order to simulate anticipated environmental conditions.
Still another object of this invention is to provide a continuous K testing device which can be utilized to test soil having been subjected to all sorts of environmental conditions such as freezing, thawing, snow, water soaking, or other conditions.
Yet another object of this invention is to monitor the Rankine stress ratio during a program of cyclical loading and unloading designed to simulate soil stress conditions that are known to occur in soils as a result of vehicular traffic or earthquakes.
One device of the prior art utilizes a stress gauge to sense the amount of lateral displacement of the receptacle. However, the device of instant invention permits direct measurement of developed lateral stress as well as direct measurement of applied vertical stress through fluid pressure. The testing device of this invention permits field conditions to be ideally simulated since the lateral elasticity is governed by compression of fluid so as to increase in magnitude as the test progresses.
A further object of the invention is to provide a testing device which is portable so that the test may be conducted at a field site on samples obtained therefrom.
A further object of the invension is to provide a testing device that directly measures the soil-to-steel coefficient of sliding friction.